Australia's greenhouse gas cut targets quietly tripled on Saturday night, from a 5 per cent cut by 2020 to a cut of more than 18 per cent.

The failure of the federal government to repeal the carbon price has triggered a far more ambitious emissions reduction target.

As of midnight May 31, the Clean Energy Act 2011 passed by the Gillard government sets a default target that is more than triple the previous 5 per cent goal.

Under the existing legislation, the government of the day was supposed to have decided on an emissions cap to enable the start of a carbon emissions market in 2015. Since no cap has been set, a default target is automatically generated.

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This goal is based on a tally that is 38 million tonnes below the 2012-13 emissions level, shrinking each year by 12 million tonnes. In practise, since 2012-13 emissions were much less than expected, the resulting trajectory is also for a lower target by the end of the decade.

The default target is 475.8 million tonnes by 2019-20, equivalent to 18.8 per cent below 2000 levels, according to the Greens, who had their estimates confirmed by the Parliamentary Library.

Greens leader Christine Milne said the measure was inserted in the act to insure against ''a government like this refusing to set a cap''.

''It won't have realised because it never put its mind to the detail,'' Senator Milne said. ''By doing nothing more than we are already doing, we are getting to 18.8 [per cent] and if we put a bit of effort in, we can go even higher.''

A spokesman for Environment Minister, Greg Hunt, declined to confirm the government had overlooked the default.

''We have always said we will repeal the carbon tax - lock, stock and barrel,'' the spokesman said. ''We will cut emissions by 5 per cent from 2000 levels by 2020, and we'll do it without a carbon tax.''

The potential for a higher Australia target comes as the US prepares to unveil a much more aggressive climate policy of its own on Monday.

President Barack Obama is expected to unveil plans to slash emissions from existing power plants by 30 per cent of 2005 levels by 2030 – a move likely to add pressure for countries like Australia and China to lift their own objectives.

Andrew Macintosh, an associate professor at the Australian National University, said the cost of cutting emissions was falling. ''The economic cost of meeting the 5 per cent target is likely to be significantly lower than previously forecast,'' Professor Macintosh said.

While the government remains confident it can secure enough votes in the new Senate after July 1 to repeal the carbon price, uncertainties will fester so long as the current tax remains.

For instance, should the debate linger to September trade-exposed industries will be eligible for many free emissions permits, said John Connor, chief executive of the Climate Institute.

“Retailers are beside themselves that they may have to rebill people,” Mr Connor said.